1. Field of the Invention
This invention relates to an ophthalmic compositions for use as a fitting or diagnostic aid, particularly for use with hydrogel contact lens.
2. Description of the Prior Art
It is well known in the prior art to use sodium fluorescein, commonly referred to simply as fluorescein, to evaluate the fit of contact lens and to evaluate the tear film and cornea prior to and following contact lens wear. Fluorescein is considered by most authorities to be an essential tool in making these evaluations. Desiccation, dry spots, epithelial defects, and certain other irregularities are difficult, if not impossible, to detect without the use of fluorescein.
Because of its absorption by hydrogel contact lens and its staining the lens, fluorescein can not be used for evaluating the fit of the hydrogel lens and, if it has been instilled for evaluation of the tear film and cornea, a certain amount of time must be allowed to elapse before a hydrogel lens can be inserted.
A one hour wait after using fluorescein is commonly recommended before such an insertion is made. This delay of an hour prior to hydrogel lens insertion is severely limiting to the practitioner and may not allow the use of fluorescein. Also, there is no direct way of evaluating the corneal-lens relationship as there is for hard lens with fluorescein. The need for a substitute fluorescein dye for use with hydrogel lens was apparent.
To overcome this problem, a new fluorescent water-soluble dye, fluorexon, was adopted for use with hydrogel lens.
Hydrophilic soft contact lens are made of polymers having a strong affinity for water. The polymeric macromolecules are interconnected by crosslinks forming 3-dimensional networks. The crosslinks render the polymer insoluble in all solvents. However, network polymers swell in good solvents, forming gels. When the swelling solvent is water, the material is termed a hydrogel. Hydrophilic soft lens are often referred to as hydrogel lens.
Hydrogel contact lens absorb aqueous solutions until swelling equilibrium is reached. The degree of swelling depends, for each given type of hydrogel contact lens, on its chemical composition, the degree of crosslinking, and on the compositon of the bathing solution.
Hydrogel lens consist of a polymer matrix containing interconnecting interstices of the network which are filled with an aqueous solution. The interconnecting interstices of the network are open to the surface of the lens, and it is proper to say that hydrogel lens are porous. Ions and molecules of dimensions smaller than the "pores" in the lens are easily absorbed into the lens. Through the pore size of the typical hydrogel lens, fluorescein can readily be absorbed. However, it was found that fluorexon was of larger dimensions and as such much less could be absorbed resulting in only negligible staining allowing its use with hydrogel lens without the accompanving disadvantages of transparency changes due to staining. The use of compounds such as fluorexon and fluorescein in these applications depends upon their ability to absorb light at characteristic wave lengths, which peaks for fluorescein at 490 nm and for fluorexon at 494 nm, and they emit light at longer wave lengths, which peaks in the case of fluorescein at 520 nm and fluorexon at 524 nm. A disadvantage in the prior art has been that a fluorescein solution 0.25% by weight in normal saline excited at wave length 490 nm, fluoresces twice as much as fluorexon of the same concentration. Because the molecular weight offluorexon (710) is almost twice as large as the molecular weight of fluorescein sodium (376), at the same weight percent concentration, almost twice as many molecules of fluorescein as fluorexon will be present in equal volume of both solutions. But, even in solutions of the same molar concentration, 0.47% by weight fluorexon and 0.25% by weight fluorescein, the fluorescence of fluorexon is still about 50% lower than the fluorescence of fluorescein. This happens because, at these relatively high levels of concentration, fluorescence does not increase linearly with increasing concentration. In other words, it is impossible to increase the concentration of fluorexon to a level where its degree of fluorescenceequals that of a 0.25% by weight solution of fluorescein.
Accordingly, while fluorexon which stains hydrogel lens slowly and gradually and is easily reversible, is preferred over fluorescein which stains hydrogel lens easily and intensively, the lower fluorescene of fluorexon has been one of the reasons for its limited adoption by practitioners in the art. Thus, a need exists for a means of increasing the relative fluorescence of fluorexon and maintaining that increased level of fluorescence even after its addition to the eye. As stated above, merely increasing the concentration of fluorexon is not a complete answer since the fluorescence does not increase linearly, fluorexon has only limited solubility in water, and because it is desirable to reduce the concentration of materials put into the eye due to the potential for irritation.